1. Field of the Invention
The invention relates to methods for measuring the plasticity of materials such as ceramic raw materials and masses. Furthermore, the invention relates to a device for implementing such methods.
2. The Prior Art
The term plasticity of a ceramic raw material or a ceramic mass is understood to mean the capacity of the ceramic raw material or mass to deform under the effect of external forces, without losing the cohesiveness of its particles. Another characteristic of plasticity is that irreversible deformation occurs only if the external force has exceeded a minimum value, known as the yield value. Forces below the yield value cause only elastic, reversible deformation.
In the case of clay/water mixtures, plastic deformation is found at water contents of approximately 20 to 25%.
The yield value has a very significant practical importance with respect to ceramics. It prevents an already shaped, but unfired ceramic piece from deforming under its own weight.
Experience has taught that plasticity increases with increasing deformation speed. This effect is utilized in practice by shaping ceramic materials at the highest possible speed.
Methods of Measuring Plasticity
For practical operations, two properties of a plastic mass are generally significant: the force that must be applied for deformation; and the maximum deformation that can be achieved until cracks occur.
The methods of measuring plasticity can be divided into methods in which a variable that is assumed to have a relationship with plasticity is measured; and methods in which an attempt is made to obtain a direct conclusion concerning plasticity.
In the Pfefferkorn method, a plate having a defined weight of 1192 g is dropped from a height of 186 mm, onto a measurement cylinder having a height (ho) of 40 mm and a diameter of 33 mm, whereby compression to height h1 occurs. The water content at which the compression ratio ho:h1=3.3:1 is considered to be the plasticity number according to the Pfefferkorn method. The water content at which a compression ratio h0:h1=2.5:1 is reached is referred to as the make-up water requirement.
In the method according to Dietzel, the same equipment used in the Pfefferkorn method is used, however rather than using a high deformation speed, the cylinder is compressed slowly, until cracks form. The compression in percent of the original height is considered to be the measure of plasticity.
In the Atterberg method, which is used internationally by soil scientists, two moisture content values representing limit values of the plastic state and the fluid state are determined. The limit value of the plastic state is the roll-out value, below which a mass can no longer be rolled out into thin strands without becoming crumbly. The limit value of the fluid state is the flow limit, at which a cut into the mass flows together when knocked on. The width of the water content range between these limit values serves as a measure of the plasticity.
Based on the Attenberg method, the plasticity number according to Rieke is considered to be the range between the roll-out limit and the make-up water requirement, which is defined to be the moisture content at which the mass just stops sticking to a person's hands.
According to Cohn, the water content at which a standardized, loaded rod penetrates to a defined depth into the mass within a predetermined time is determined.
Norton and Baudran use a torsion test for determining a measurement number for plasticity. The product of the yield value and the maximum deformation is referred to as “workability.” It reaches a maximum at a certain water content.
Haase uses the quotient of tear resistance and deformation pressure as a measure of plasticity. Haase's deliberations proceed from the assumption that a mass is more plastic the greater the cohesion of the particles (tear resistance), but the smaller the forces for reciprocal shifting of the particles, (the lower the viscosity of the mass).
The method according to Hofmann and Linseis is based on a similar consideration. Here, the quotient of the tear resistance and the yield value is used as a measure of plasticity. The yield value is characterized by the press-out pressure at which the mass can be transported through a die.
In the determination of plasticity using the Brabender plastograph, a powder is thoroughly mixed in a kneading chamber at a constantly increasing water content. The torque of the drive motor of the kneading arms, i.e. the resistance of the powder/liquid system to deformation, is measured. The amount of torque at the curve maximum as well as the steepness of the flanks of the maximum are considered to be a measure of the plasticity. Furthermore, the water content at the maximum torque can be derived from the curve.
A cyclical stress test (torsion) is indicated by Ashbury. Here the effective stress varies between maximum values +t and −t with a period duration of approximately 1 min. The results are quite informative for a theoretical discussion of plasticity. The deformation e that occurs is measured and the area of the hysteresis curves is a measure of the deformation work to be exerted.
The informational value and acceptance of the foregoing methods are limited, in part, by a great amount of measurement technology effort and, in part, by limited accuracy and lack of personal neutrality. The elastic properties of the sample are not determined, with the exception of the Brabender plastograph (measurement kneader).